(1) Field of the Invention:
This invention relates to a method for the production of an expansion-molded article of a polypropylene resin, which method allows to obtain the expansion-molded article with good quality without need for any pretreatment to build up an internal pressure within each pre-foamed particle even when the internal pressure of the pre-foamed particles is substantially equal to the atmospheric pressure.
(2) Description of the Prior Art:
Numerous research has been conducted to obtain molded articles, which are in conformity with molds, by using pre-foamed particles of polyolefin resins. Such research has lead, for example, to techniques disclosed in Japanese Patent Laid-Open Nos. 34458/1972, 85158/1974 and 128065/1974. Each of these techniques includes as an essential requirement an additional supply of gas (organic gas, inorganic gas or the like) to pre-foamed particles of a crosslinked resin and subsequent expansion molding of the resultant pre-foamed particles upon production of an expansion-molded article from the pre-foamed particles.
Polyolefin resins are considerably different from polystyrene-base resins in gas barrier properties, viscoelastic characteristics or fluid characteristics upon melting, etc. and involve such problems that the gas in cells is immediately allowed to dissipate and loses its expanding ability, molded articles undergo shrinking, and/or the fusion-bonding among particles cannot be achieved sufficiently. As a method for overcoming such problems, it is necessary to feed an additional supply of organic gas or inorganic gas (e.g., air) to pre-foamed particles. A technical field different from that of polystyrene resins has therefore been established for polyolefin resins although they are both directed to steam chest molding of beads. It has also been known to mold ethylene resins without applying any pretreatment to build up an internal pressure therein, including the invention described in Japanese Patent Publication No. 7816/1980 for example.
The feeding of an additional supply of organic gas or inorganic gas to pre-foamed particles results in considerable loses in both equipment and cost. Its omission has hence remained as the greatest dream and at the same time the most difficult subject for those dealing with the present field of art.
For example, the invention described in Japanese Patent Laid-Open No. 34458/1972 requires a large expense for a gaseous foaming agent. On the other hand, the invention described in Japanese Patent Laid-Open No. 128065/1974 requires substantial initial cost for maintenance and safety equipment and handling system for high-temperature and high-pressure gas. The invention of Japanese Patent Laid-Open No. 85158/1974 is accompanied by such a drawback that the resulting molded articles are not stable in quality. Moreover, pre-foamed particles imparted with expanding ability by additional supply of gas as in these techniques have a common problem that their expanding ability does not last long. They are hence accompanied by a problem that excellent molded articles cannot be obtained unless a special consideration is exercised to consume pre-foamed particles within a very short period of time subsequent to the additional supply of gas, for example, by conducting the feeding step of the additional gas and the molding step continuously. They are by no means considered to have been completed to such an extent that many molding companies can produce molded articles easily from pre-foamed particles furnished by pre-foamed particles makers.
Furthermore, no promising effects were obtained from an application of the technique of the invention described in Japanese Patent Publication No. 7816/1980 to the production of expansion-molded articles of a polypropylene resin.